As a continuously variable transmission to be provided on a vehicle, there is known a continuously variable transmission of a belt type that includes a primary pulley, to which a drive force of an internal combustion engine is transmitted, a secondary pulley coupled to a wheel, and a belt wound upon the pair of pulleys and serves to change the winding radius of the belt in each of the pulleys, thereby varying the transmission gear ratio continuously and steplessly.
A control device for controlling such a continuously variable transmission of a belt type changes hydraulic pressure in a hydraulic chamber provided in each of the pulleys to vary the groove width of each of the pulleys to which the belt is fitted, thereby changing the winding radius of the belt in each of the pulleys to control the transmission gear ratio.
More specifically, the control device raises the hydraulic pressure in the hydraulic chamber of the primary pulley to decrease the groove width of the primary pulley when the transmission gear ratio is reduced. Furthermore, the control device correspondingly reduces the hydraulic pressure of the hydraulic chamber of the secondary pulley to increase the groove width of the secondary pulley. Consequently, the winding radius of the belt in the primary pulley is increased, while the winding radius of the belt in the secondary pulley is reduced in a state in which the tension of the belt is ensured. As a result, it is possible to reduce the transmission gear ratio while suppressing a slip of the belt in each of the pulleys.
Moreover, the control device reduces the hydraulic pressure in the hydraulic chamber of the primary pulley to increase the groove width of the primary pulley, and raises the hydraulic pressure in the hydraulic chamber of the secondary pulley to reduce the groove width of the secondary pulley when the transmission gear ratio is increased. Consequently, the winding radius of the belt in the primary pulley is reduced, while the winding radius of the belt in the secondary pulley is increased in the state in which the tension of the belt is ensured. As a result, it is possible to increase the transmission gear ratio while suppressing slip of the belt in each of the pulleys.
The control device for a continuously variable transmission to be provided on a vehicle sets a target transmission gear ratio depending on a depression amount of an accelerator pedal, a vehicle speed, an engine speed or the like, and controls the hydraulic pressure in the hydraulic chamber of the pulley based on the target transmission gear ratio. The control device calculates the actual transmission gear ratio based on the rotational speed of the primary pulley and that of the secondary pulley when the hydraulic pressure in the hydraulic chamber of the pulley is controlled, and feedback controls the hydraulic pressure in the hydraulic chamber of the pulley based on the difference of the actual transmission gear ratio from the target transmission gear ratio.
In a case in which the control device includes a rotational speed sensor of an electromagnetic pickup type as a rotational speed sensor for detecting a rotational speed of a power transmission system in order to detect the rotational speed of the pulley, precision in detection is reduced or the detection itself of the rotational speed cannot be carried out when a rotational speed of a detection target is low du to the characteristics of the sensor. For this reason, when the rotational speed of the detection target is reduced significantly as in a state immediately before stopping of the vehicle, the rotational speed of the pulley cannot be detected accurately, and the actual transmission gear ratio cannot be calculated precisely. As a result, a proper feedback control cannot be executed, and a gear change control might be unstable.
In the control device for a continuously variable transmission described in Patent Document 1, therefore, when it is estimated that the vehicle speed is less than the reference vehicle speed, and the rotational speed of the secondary pulley cannot be detected accurately by the rotational speed sensor, hydraulic oil is removed from the hydraulic chamber of the primary pulley to reduce the hydraulic pressure in the hydraulic chamber of the primary pulley.
If the hydraulic oil is removed from the hydraulic chamber of the primary pulley, the primary pulley is expanded by the tension of the belt, and the groove width of the primary pulley is maintained at the maximum groove width within a changeable range. By employing a configuration for removing the hydraulic oil from the hydraulic chamber of the primary pulley when estimating that the rotational speed of the secondary pulley cannot be detected accurately as described above, therefore, it is possible to maintain the transmission gear ratio at the maximum transmission gear ratio also in a situation in which a proper feedback control cannot be executed.